In general, noise that is present in a listening environment nearly always compromises the experience of listening to audio through headphones. For instance, in an airplane cabin, noise from the airplane produces unwanted acoustic waves, i.e., noise, that travel to the listener's ears, in addition to the audio program. Other examples include computer and air-conditioning noise of an office or house, vehicle and passenger noise in public or private transportation, or other noisy environments.
In an effort to reduce the amount of noise received by the listener, two major styles of noise reduction have been developed, passive noise reduction and active noise cancellation. Passive noise reduction refers to a reduction in noise caused by placing a physical barrier, which are commonly headphones, between the ear cavity and the noisy outside environment. The amount of noise reduced depends on the quality of the barrier. In general, noise-reduction headphones having more mass provide higher passive noise reduction. Large, heavy headphones may be uncomfortable to wear for extended periods, however. For a given headphone, passive noise reduction works better to reduce the higher frequency noise, while low frequencies may still pass through a passive noise reduction system.
Active noise reduction systems, also called active noise cancellation (ANC), refers to the reduction of noise achieved by playing an anti-noise signal through headphone speakers. The anti-noise signal is generated as an approximation of the negative of the noise signal that would be in the ear cavity in absence of ANC. The noise signal is then neutralized when combined with the anti-noise signal.
In a general noise cancellation process, one or more microphones monitor ambient noise or noise in the earcups of headphones in real-time, then generates the anti-noise signal from the ambient or residual noise. The anti-noise signal may be generated differently depending on factors such as physical shape and size of the headphone, frequency response of the speaker and microphone transducers, latency of the speaker transducer at various frequencies, sensitivity of the microphones, and placement of the speaker and microphone transducers, for example. The variations in the above factors between different headphones and even between the two ear cups of the same headphone system mean that that optimal filter design for generating anti-noise also vary.
Currently no Active Noise Cancellation system exists that can efficiently accommodate all of the variable factors to be considered when generating the anti-noise signal. For instance, digitizing the microphone signals and processing the signal at normal audio rates introduces large latency. Because the ANC performance depends on the ability to detect noise and produce the anti-noise signal soon enough in time to cancel the noise, a large latency is detrimental to ANC performance.
Embodiments of the invention address this and other limitations of the prior art.